This document explains how Continuity Testing (COT) works for the Cisco
PGW 2200 in configured signaling mode. COT is an automated diagnostic procedure
performed in the Signaling System 7 (SS7) Public Switched Telephone Network
(PSTN) between switches to ensure that circuits are in service and not
experiencing excessive signal loss. This validation is performed by sending
tones across the bearer channel. It is performed on a sampling basis. For
instance, on 10 percent of the calls (in the Cisco PGW 2200, the sampling rate
is defined by CotPercentage).

The information in this document is based on the Cisco PGW 2200
Softswitch.

The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.

Since the bearer path is distinct from the signaling path, SS7 requires
specific means to validate the bearer channel before the call is established.

COT is requested by the Cisco PGW 2200 originating as an IAM request or
as a separate SS7 ISUP message using the Continuity Check Request (CCR) or
Circuit Reservation Message (CRM). Two types of COT are Loopback and
Transponder (Tone check) methods (see the table).
The destination Cisco PGW 2200 places the related circuit in either Loopback
mode 2010 Hz or in Transponder mode 1780 Hz (see the table). The Loopback form of COT is used on 4-wire trunks.
In the 2-wire case, when the originating switch sends a tone, the receiving
switch sends a different frequency tone in response. This is the Transponder
mode of COT. If the circuit testing fails during COT, alarm information is
generated allowing diagnostic software to busy out the trunk, which can be
checked with the man-machine language (MML) command
rtrv-alms. Calls are prevented from being routed to
the circuit and are COT failed.

The Cisco PGW2200 software application performs the signaling and/or
call control tasks. This section will talk about the signaling mode. The Cisco
PGW 2200 sends NI2+ commands to NAS to transmit and detect tones or loopback
circuit.

Look at an SS7 sniffer trace (Cisco Snooper PT-MTC) for an SS7 IAM
message. This is used to check the message into the IAM on
CONTINUITY CHECK IND which has a value equal to
1. This indicates that you need to do a CCR
required on this circuit.

Note: No test is performed in case the value equals 0 (0
Continuity_check_not_required).

You can check the status on the network access server (NAS) ISDN
interface channel using the command show isdn
service. You can also check the status of the B-channel which is
currently sending the COT testing into state 5 [ Maitenance
Pending].

If you want to check the configuration using MML, run the command
prov-rtrv:sigsvcprop to find out the COT settings.
You need to edit the properties.dat using MML commands to set-up the method
(Loop or Tone) and tones (2010 and 1780) correctly.

Note: The two values that are provisionable are 1780 and 2010 for the COT
tones.
Table
4-4: Provisionable Properties lists the properties that can be
provisioned and indicates if the modified property value takes effect without
stopping and restarting the Media Gateway Controller software.

On the NAS, no COT configuration task is requested. Please be aware
that for Nailed solution you are using ISDN switch-type isdn
switch-type primary-ni2c between the PGW 2200 and NAS.

If you run into a COT test failure, this can be viewed on the Cisco PGW
2200 in a variety of ways. The simplest is to use the MML utility and the MML
command rtrv-alms:cont to receive continuous alarm
updates.

Note: From Cisco PGW 2200 release 9.4(1)
clr-alms is removed from the MML command (refer to
Cisco bug ID
CSCdy63984
(registered customers only)
for further information). You can first
retrieve the current alarm status using rtrv-alms.
Entering help at the
mml> prompt provides you with a listing of
all MML commands.

Attempt to put the Circuit Identification Code (CIC) back into
service (the remote end may not comply).

On the Cisco PGW 2200, use the MML command
unblk-cic:<point code>:CIC=<number>. You
can then retrieve the CIC status by using the MML command
rtrv-cic:<point code>:CIC=<number> or
use the rtrv-tc:all command.

On the NAS, use the configuration command isdn
service dsl <dsl #> b_channel <channel #> state <0 = In
service> available under the interface
Serial0:15 interface command. You can then retrieve the B-channel
status using the show isdn service command at the
console log.

For example:

AS5300(config-if)#isdn service dsl 0 b_channel 1 state 0

Next, you can also run a manual COT testing again. Manual COT tests
can be initiated on the Cisco PGW 2200 without requiring the SS7 network to
initiate a COT test.

For example, on PGW2200a:

mml> tst-cot:<point code>:CIC=<number>

If you run into COT problems, issue the debug isdn
q931 command on the NAS, as shown in this figure.

This figure xplains the MML tst-cot
command and the CCR sent out to the SS7, including the debug isdn
q931command output.

This section contains the MGCP COT callflow for the PGW 2200 Call
Control situation. The Media Gateway Control Protocol (MGCP) organizes the
communication between a Media Gateway controller [Cisco PGW2200] and a Media
Gateway [Cisco AS5xxx]. This section shows Cisco snooper trace detail for a
hairpinning call with loopback COT' and a manual MML cot test from the PGW
2200.

In this figure, COT is initiated by the PSTN switch that sends the IAM
request. This PSTN switch reports the test result using a SS7 COT message if
successful or a SS7 Release message if failed. The Cisco PGW 2200 puts the
related circuit either in loopback mode or in transponder mode. The Cisco PGW
2200 that receives an IAM requesting COT (or a CCR message) first checks for
the ExtCOT parameter in the /opt/CiscoMGC/etc/properties.dat file.

Upon recept of the SS7 IAM message with the CONTINUITY
CHECK IND = 1 which means
Continuity_check_required_on_this_circuit, the PGW 2200
recognizes that a continuity test has been requested. It immediately sends a
CRCX CreateConnection message to the Cisco Gateway with a M:
loopback to put a loopback on the incoming trunk.